The nearly seven-hour passage of the planet between the Earth and the Sun will be seen as a black dot on the solar surface, but should only be viewed through approved solar filters to avoid the risk of blindness, experts warn.

In Australia the event started at about 8.15am (AEST) this morning and will continue for six-and-a-half hours.

Astronomers say the safest way for people to watch the event is from a public observation site.

All of the transit will be visible in East Asia and the Western Pacific, including Australia.

Most of South America as well as west and south-west Africa will not get to see it except by broadcasts from other locations.

'Wonderful and rare'

Important: Do not look directly at the sun or view the transit using sunglasses, smoked glass or reflective surfaces such as DVDs.

The US space agency NASA has promised "the best possible views of the event" through high-resolution images taken from its Solar Dynamics Observatory (SDO), in orbit around the Earth.

"A transit is a wonderful and rare sight; when you consider the vastness of the sky, for a planet to pass the disc of the Sun is pretty unusual - and you do have to wait until 2117 for the next one," SDO co-investigator Richard Harrison said.

The European space agency's Venus Express is the only spacecraft orbiting the hot planet at present and will be using light from the Sun to study Venus's atmosphere.

ESA and Japan's space agency also have satellites in low-Earth orbit to observe as Venus passes in front of the Sun.

And the NASA Hubble Space Telescope, which cannot view the Sun directly, will use the Moon as a mirror to capture reflected sunlight and learn more about Venus's atmosphere.

Many universities and observatories have also scheduled viewings and astronomy talks for the public.

For the smartphone set, there is an app for tracking the transit, according to Astronomers Without Borders.

Scientists say that studying the transit will boost future efforts to identify distant planets and learn more about their atmospheres.

Only six Transits of Venus have ever been recorded. There have been 53 transits of Venus across the Sun between 2000 BC and the last one in 2004.

Sunglasses are not strong enough to shield the eyes for viewing the transit, experts say.

Specialised solar filters and glasses will be offered for sale at some observatories, or people can make a pinhole camera or use a pair of binoculars - not held to the eyes - to project the Sun's image onto a light-coloured surface.